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The Imp of the Perverse by Edgar Allan Poe.

The last of this collection's unreliable narrators. Again, he is a murderer, but here, unlike stories such as The Tell-Tale Heart or The Cask of Amontillado, he is not intent on proving his sanity. This narrator gets around to telling his story at the end, spending most of the tale in explanation of his theories on the workings of the mind.